December 2, 1892.] 



SCIENCE. 



3^5 



external conditions, or from the opposite position of those who 

 aver it to have been designedly contrived to meet the special re- 

 quirements of those conditions, it is a matter for surprise that any 

 should have been found to express a belief that, for distinctness of 

 vision and other purposes for which eyes are required, these 

 specialized and elaborate contrivances are little better than optical 

 failures. Such a notion, if capable of proof, would be a unique 

 exception to that perfect adaptation of means to ends, which, 

 wherever our knowledge is complete, we find everywhere else in 

 nature. 



Apart from the question as to whether the nervous structure of 

 an insect's eyes enables it to utilize rays which are beyond the 

 compass of our own, it is clear that the nature of light requires 

 in all organs of vision a structure which is analogous in its optical 

 principles; that is, tht-re must be the means of forming an image, 

 a sensitive screen upon which to receive it, and a connecting line 

 along which the received vibrations may be conveyed to the ulti- 

 mate seat of the sensorial impressions. Hence we find a lens, a 

 retina, and an optic nerve to be common to all. We may also 

 infer that the external physical requirements will be approximately 

 the same, so that the vibrations must be of proper quality, they 

 must be of sufficient intensity, and they must impinge upon the 

 retina for a sufficient time to enable its sympathetic fibres to re- 

 spond to and take up the impulses imparted. 



The first difficulty which we meet with in approaching the sub- 

 ject is one which does not apply to insects alone, and therefore 

 does not enter exclusively into present considerations. 



In the case of human vision the optic angle is so small that each 

 eye sees the same object, indeed confusion is experienced and a 

 double image is perceived unless the optic axes are so converged 

 upon tbe object as to briuj its image upon the correspondingly 

 sympathetic portions of each retina. But in the case of some 

 animals, and in that of birds, the increase of the optic angle pre- 

 cludes the possibility of such co-ordination, so that an entirely 

 dissimilar picture is presented to each eye, and a further compli- 

 cation is introduced in the case of the chameleon, whose eyes are 

 capable of independent movement in every direction within the 

 limitations of their sockets. We are unable to realize in our own 

 minds what the effect of this may be, because, with the exception 

 of impressions received through the sense of touch, we have no 

 analogous experience, but we may readily conceive it to be a 

 matter of interpretation by which the wide extension of the visual 

 field induces the perception of a panoramic view of the surround- 

 ings; and if to eyes which are laterally situated we add also others 

 on the vertex, with divergent axes as we find in the ocelli of 

 many insects, we may further imagine that an extension of the 

 panorama vertically may present a picture embracing an area of 

 more than half a hemisphere. 



But when we come to regard vision by means of compound 

 eyes, such as we find in insects, other considerations present them- 

 selves and it is obvious that the question as to " why and where- 

 fore " requires another answer. I should like to be allowed here 

 to make a protest against the continued application of the term 

 "facetted" to the corneal surface of the compound eye, as con- 

 veying an idea which is not strictly correct. At a recent conver- 

 zatione I found, amongst other objects exhibited, a plano-convex 

 lense, the curved surface of which was ground off into numerous 

 actual facets, and visitors were invited to look through this from 

 its plane face in order to realize the effect produced by the 

 "facetted" eye of an insect. I need not point out that both 

 structurally and optically this conception was entirely erroneous. 

 The structure of the compound eye is, however, now so well 

 known that I do not propose to enter upon it here at any length, 

 but vvill merely refer to the recent researches of Professor Exner 

 and others as showing (1) that, contrary to previous speculations, 

 it is capable of forming a distinct image of considerable ampli- 

 tude, towards which each ocellite contributes its share; (3) that 

 in the picture so produced very many of the pictures formed by 

 adjacent ocellites are either superposed or overlap each other in 

 such a way that tbe corresponding portions of each become coin- 

 cident upon the retina; and (3) that it is highly probable that the 

 structure of the organ provides an arrangement which serves a 

 purpose equivalent to that of the iris in the vertebrate eye, with 



the further suggestion of a means of focussing. Professor Ex- 

 ner's experiments also prove that by the intervention of the crys- 

 talline cones this composite, or " summation," image is erect, and 

 is formed at an increased distance from the corneal surface. 



Those who have access to the last edition of the late Dr. W. B. 

 Carpenter's book, " The Microscope and its Revelations," will have 

 noticed a reference to these researches, but it may be as well to 

 note that tbe figure on page 908 appears to have been laterally in- 

 verted by the engraver, my own recollection and a rough sketch 

 taken at the time enabling me to say that in the original photo- 

 graph the letter R was not reversed as shown in the wood cut, 

 and the church faced the other way. 



Assuming, therefore, that distinct and otherwise perfect vision 

 is enjoyed by the possessors of compound eyes, it is reasonable to 

 suppose that, if we desire to know what is the raison d'etre of 

 their complex structure, we shall be most likely to find the an- 

 swer, if we proceed upon lines indicated by the further assump- 

 tion, that it is I'equireri to meet some special necessity arising from 

 conditions of life which differ from those of other creatures. 



Pursuing the inquiry in this direction the following considera- 

 tions make it probable that such conditions may be recognized in 

 connection with the extremely rapid movements of insects in 

 flight. 



The angular diameter of the field of distinct vision in the human 

 eye (as distinguished from the visual angle) is much smaller than 

 is commonly supposed, experiment shows that it varies with in- 

 dividuals, but, for present purposes of illustration, we will call it 

 10°. The inconvenience which would otherwise arise from so 

 circumscribed an area is in practice largely compensated for by 

 the celerity and freedom of motion common to the eyes and head, 

 by virtue of which also we are able to neutralize the effect of our 

 own movements, and, within certain limits, to perceive moving 

 objects which would otherwise cross the field in less time than 

 the minimum required for the production of a distinct retinal 

 image. The exact duration of this period is a matter of personal 

 equation, but may usually be taken as about -l^ of a second. Now 

 it is a matter of common experience that when travelling in a 

 railway train at the rate of, say, fives miles an hour, we can, with 

 fixed vision, clearly distinguish the flowers growing adjacent to 

 the track, but, as the speed increases, we become less able to do 

 so, until, at 50 miles an hour, they cross the visual area too 

 rapidly to leave more than an indistinct impression of horizontal 

 lines. It is, however, conceivable that if, as soon as an object 

 had traversed the field of one lens, it came successively within 

 the scope of nine others, which, without break of continuity, 

 would project its image upon the same portion of the retina, the 

 persistence of the image would be increased tenfold, with the ob- 

 vious result that the flowers would then be seen as clearly whilst 

 passing them at 50 miles per hour as they would be under ordi- 

 nary circumstances at one-tenth the speed. 



If there is truth in this suggestion, that the use of compound 

 eyes is to enable their possessors to enjoy distinct vision during 

 rapid flight, it would appear to derive support from the fact that 

 we find, as a rule, that in larvae and in insects which are wingless 

 the eyes are either simple, or that the ocellites, of which they are 

 compounded, are comparatively few in number; whilst in those 

 with wings the compound character is developed to its highest 

 degree in genera whose powers of flight are most remarkable. 

 Instances are not wanting in which the eyes of apterous females 

 are simple, whereas they are compound in the case of the winged 

 males of the same species. 



That such extremely rapid flyers as the dragon-flies and preda- 

 tory DIptera are endowed with acute and accurate powers of 

 vision seems to require no further proof than is afforded by the 

 unerring manner in which they strike and capture other insects 

 which are also on the wing. 



Professor Sophds Ruge of Dresden, an authority on matters 

 relating to the discovery and exploration of America, pronounces 

 Mr. Winsor's "Columbus" "the most important contribution 

 that North America has made to the present commemoration " of 

 1492. 



